At the Council of Chalcedon in 451, when bishops from throughout the Mediterranean world gathered in Constantinople, Emperor Marcian asked the Patriarch of Jerusalem to bring the relics of Mary to Constantinople to be enshrined in the capitol. The patriarch explained to the emperor that there were no relics of Mary in Jerusalem, that "Mary had died in the presence of the apostles; but her tomb, when opened later . . . was found empty and so the apostles concluded that the body was taken up into heaven."

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Perhaps the most famous of these traditions is a brief work known as the Euthymiac History. This legend was interpolated into the second of John of Damascus’ homilies on the Dormition* at an early point in their transmission, where it is identified as a quotation from ‘the third book of the Euthymiac History, chapter 40’. Its inclusion in these, ‘the most celebrated of all the ancient homilies for the feast of the Dormition’, ensured that it was the most widely circulated of the late apostle traditions.150 The Euthymiac History, as quoted in John’s homily, describes an incident that is supposed to have occurred during the events of the council of Chalcedon. While Juvenal[, the Patriarch of Jerusalem,] and the other bishops of Palestine were present for the council, the imperial couple, Pulcheria and Marcian, enquired about the relics of the Virgin Mary, asking that Mary’s remains be sent to the imperial capital, in order to protect it. In response, Juvenal briefly narrates the events of Mary’s Dormition, explaining why there are actually no bodily relics to be had. There is, however, he tells them, another kind of relic that he could send. Three days after Mary’s burial, Juvenal explains, the apostle Thomas finally reached Jerusalem, and, having missed the events of Mary’s Dormition, he requested that the tomb be reopened, so that he might pay his respects (not out of doubt, I might emphasize). When the apostles opened the tomb, they were startled to find no body, but instead only Mary’s funeral robe. Juvenal then concludes by referencing the passage from Ps.-Dionysius’ The Divine Names discussed above, after which the imperial couple requests that Juvenal send them the garment. When Juvenal returns home to Jerusalem, he fulfils their request, and Marcian and Pulcheria enshrined the robe in the church of Blachernae, the Constantinopolitan church that housed this famous relic. It is difficult to date this tradition, and we do not know whether it arose sometime before or after the Islamic conquest. We know only that the legend developed sometime between 550 and 750, making it a potential witness to the earliest development of the Dormition traditions.

*St. John Damascene's homily is referenced in Pope Pius XII's 1950 definition of the dogma of the Assumption, Munificentissimus Deus, §§21-22.

@SolaGratia It's interesting that the Euthymiac History seems to predate St. John Damascene's homily, which was one of the oldest testimonies Pope Pius XII mentioned in Munificentissimus Deus.
– GeremiaApr 21 '18 at 22:26

John of Damascus records the source of the quote as from St. Juvenal bishop of Jerusalem CE 450.

"St. John Damascene (d. 749) also recorded an interesting story concerning the Assumption: "St. Juvenal, Bishop of Jerusalem, at the Council of Chalcedon (451), made known to the Emperor Marcian and Pulcheria, who wished to possess the body of the Mother of God, that Mary died in the presence of all the Apostles, but that her tomb, when opened upon the request of St. Thomas, was found empty; wherefrom the Apostles concluded that the body was taken up to heaven." In all, the Patristic Fathers defended the Assumption on two counts: Since Mary was sinless and a perpetual virgin, she could not suffer bodily deterioration, the result of Original Sin, after her death. Also, if Mary bore Christ and played an intimate role as His mother in the redemption of man, then she must likewise share body and soul in His resurrection and glorification." - The Assumption of Mary